We do not have a good understanding of why so few (~6-7%) flowering-plant species are dioecious, despite an apparently high number of origins across angiosperm phylogeny (hundreds, perhaps thousands of transitions to dioecy). This discrepancy may reflect lower diversification rates in dioecious than cosexual lineages, or frequent reversals to cosexuailty. Reconstructions of sexual-system shifts may indicate whether instances of dioecy represent recent transitions (originating convergently on/near tree tips), or older retained primitive states, and whether there are frequent reversions of dioecy to cosexuality. Here we use a plastid phylogenomic approach to reconstruct sexual-state transitions in Hydatellaceae (Nymphaeales), in the broader context of related lineages including Amborella, Nymphaeales, Austrobaileyales (= ANA taxa) and beyond. All 13 described species of Trithuria (Hydatellaceae) are included, nine of which are cosexual; four dioecious species in the family are distantly related to each other. A microsatellite study of a species complex that includes two widespread cosexual species (T. submersa, T. bibracteata) and a geographically limited dioecious species (T. occidentalis) supports the existence of additional cosexual species in the complex. The plastid tree recovers T. occidentalis as being nested among cosexual taxa in this complex, overturning earlier phylogenetic inferences based on more limited data. Parsimony reconstructions imply multiple origins of dioecy across Hydatellaceae and relatives, but likelihood reconstructions and stochastic mappings indicate broad ambiguity in ancestral states across angiosperm phylogeny. Nonetheless, at least two of four instances of dioecy in Hydatellaceae appear to have evolved both independently and recently.